1. Field of the Invention
This invention relates to the preparation of polymer foams and is more particularly concerned with polyisocyanurate foams which have a high degree of fire resistance and generate low smoke levels during combustion.
2. Description of the Prior Art
Cellular polymers in which the major recurring polymer unit is isocyanurate, are well known and widely used in the art; see for example U.S. Pat. Nos. 3,516,950, 3,625,872, 3,644,232, 3,676,380, 3,725,319, 3,745,133, 3,763,057, 3,793,236, 3,799,896, 3,849,349, 3,876,568, 3,891,579, and 3,909,465. Generally speaking, the polyisocyanurate foams prepared in accordance with the prior art are found to be more fire resistant, in comparison to polyurethane foams, when subjected to the various testing procedures known and used in the foam art. Unfortunately, smoke generation during combustion of such cellular polymers has remained a problem. Prior art methods for lowering the smoke have included the addition of various classes of adjuvants, including inorganic fillers, graphite powder, various types of fluoroborate salts, sulfur, and the like. The use of these methods add to the cost of manufacturing the foams and, generally speaking, detract in one way or another from their physical properties. Furthermore, U.S. Pat. No. 3,644,232 discloses that foams prepared from commercially available polymethylene poly(phenylisocyanates) pop and burst into fragments when subjected to a flame and give rise to a lot of smoke and noxious fumes.
British Patents 1,223,415 and 1,404,822 describe the use of minor amounts of polyester polyols, prepared from aliphatic dicarboxylic acids and a broad class of aliphatic polyols, in a process for producing polyisocyanurate foams with minimum brittleness in addition to the expected fire retardent properties.
We have now found, quite unexpectedly, polyisocyanurate foams which possess a high degree of fire resistance and generate very low smoke levels upon combustion, in the absence of any additional smoke, or fire retardant additives. This advantageous feature is achieved by the inclusion in the foam formulations, which call for a polymethylene poly(phenylisocyanate), of a minor amount of a polyol selected from a very narrow class of polyols, heretofore not recognized by the prior art references as possessing this ability. The foams are further characterized by good physical properties.